1. Field of the Invention
The present invention relates to an internal mixer for mixing rubber, plastics, etc. with various kinds of additives, and more particularly to an internal mixer having two rotors which can be operated both at the same speed and at different speeds.
2. Description of the Prior Art
This type of internal mixer in conventional use is shown in FIGS. 3 and 4.
In the internal mixer shown in FIGS. 3 and 4 (a drawing taken along the arrow A in FIG. 3), a mixing chamber 33, together with a floating weight 31 and a drop door 32, forms communicating cylindrical closed spaces which are round in cross section; within this mixing chamber 33 are juxtaposed two rotors 38 and 39, in a non-engaged condition, which are connected with two output shafts 35 and 36 of a speed reducer 34 through a coupling 37. As a means for driving the rotors, driving power from a prime mover 40 such as a motor is transmitted to an input shaft 41 of the speed reducer 34 through a coupling 42, thus turning these two rotors 38 and 39 in opposite directions of rotation. The rotors 38 and 39 are provided with long blades 38a and 39a and short blades 38b and 39b, that is, with two blades each. These blades 38a, 38b, 39a and 39b spirally extend around the shafts of the rotors 38 and 39; these long blades 38a and 39a are curved in the opposite direction of the short blades 38b and 39b. The floating weight 31 forming a part of the upper part of the mixing chamber 33 is installed on a piston shaft 43, and vertically movably inserted in a hopper 44. The drop door 32 forming a part of the lower side of the mixing chamber 33 is designed to be opened and closed so that a mixed compound can be taken out of the mixing chamber. A numeral 45 refers to a charging port for charging a specific amount of materials to be charged plus the additives into the hopper 44.
Next, the operation of the internal mixer of the above-described constitution will be explained. A specific amount of materials, such as rubber, plastics, etc., and various additives, are charged into the hopper 44 from the charging port 45, and forced into the mixing chamber 33 by a push of the floating weight 31. Then, with the mixing chamber 33 closed, the driving power of the prime mover 40 is transmitted to the speed reducer 34 to turn the rotors 38 and 39 in opposite directions. In the mixing chamber 33, the materials to be mixed flow in the direction of the axis of the rotors 38 and 39 while being sheared at a tip clearance between the tips of the blades of the rotors 38 and 39 and the inner wall of the mixing chamber 33, thus performing so-called micro dispersion that the additives are dispersed into the materials to be mixed. Since the rotor 38 and the other rotor 39 rotate in opposite directions, the materials being mixed flow between the right and left mixing chambers. The materials being mixed, as shown at B in FIG. 4, gradually flow while swirling largely on the whole within the mixing chamber, with the additives being macro-dispersed for uniform mixing into the materials under mixing. When mixing in the mixing chamber 33 is finished, the drop door 32 located in the lower part of the mixing chamber 33 is opened to discharge the mixed compound out of the machine, thus completing one cycle of mixing operation.
The internal mixer comes in two types: one is the engaged type where two rotors are juxtaposed in an engaged condition, and the other is the non-engaged type where the two rotors are juxtaposed in a non-engaged condition and rotate at different speeds, for the purpose of improving the micro and macro dispersion functions by periodically changing the phases of the blades of she two rotors 38 and 39. However, if the two rotors 38 and 39 are rotated at different speeds, there takes place a difference in a mixing energy to be added to the materials to be mixed in the right and left mixing chambers, resulting in inactive flow of the materials between the right and left mixing chambers and accordingly in insufficient macro dispersion. In this case, the mixed compound will become of nonuniform quality. To cope with this sort of problem, there has been disclosed (refer to Japanese Patent Publication No. Hei 2-33871) an internal mixer with two rotors arranged in specific phases and driven at the same speed. According to this internal mixer, as the two rotors rotate at the same speed, there will occur no difference in the mixing energy to be added to the materials in the right and left mixing chambers, resulting in active flow of the materials between the right and left chambers and sufficient macro dispersion of the additives into one materials.
In the prior-art non-engaged type of internal mixer described above, when the two rotors are rotated at the same speed in an attempt to activate material flow between the right and left mixing chambers, an excellent mixing result is gained as compared with rotor operation at different speeds on an average. However, rotor operation at different speeds is in some cases relatively superior, depending on the materials to be mixed, in the respect of material flow characteristics and the speed of feed of the materials into the mixing chambers after a charge. It will become necessary to experimentally test the suitability of the same or different speeds of the two rotors to the mixing of materials. Therefore, the two rotors have to be driven at the same speed or at different speeds according to the materials to be mixed, and, inefficiently, gears of the speed reducer or the speed reducer itself must be changed every time the rotor speed is changed. It has been a practice to employ two types of mixers according to materials to be mixed: one with two rotors driven at the same speed and the other with two rotors driven at different speeds. However, it has been necessary to produce a mixed compound on small lots in order to produce several types of mixed compounds in small quantities. For this purpose, it is desired that there be adopted one mixer equipped with two rotors which can be driven both at the same speed and at different speeds according to a difference in the type of materials to be mixed. To meet this, it is supposed to use two sets of motors for a single mixer. In the mixer, two rotors of this mixer are connected to the output shaft of the speed reducer coupled with each of the two motors, so that the two rotors of the single mixer can be operated both at the same speed and at different speeds by changing the speed of one of the motors. In this case, the mixer has the following problem that the adoption of the two motors and two speed reducers will increase cost and moreover it is difficult to electrically exactly match up the speeds of the two motors, resulting in a phase shift.